The development of power brake systems makes it possible to offer handling of the brake system that is safe and convenient for the driver. In the case of these brake systems, the driver does not build up the pressure in the brake circuits or wheel brakes directly by his physical strength. Instead, his braking demand is usually introduced into a simulator unit with a brake pedal and this demand is correspondingly detected by sensors. It is used for calculating a setpoint brake torque in an electronic control and regulating unit, the control and regulating unit then activating the actuating unit on the basis of manipulated variables corresponding to the setpoint brake torque for building up brake pressure. The actuating unit in this case comprises for example an electric motor, which operates a tandem brake master cylinder, to the chambers of which the brake circuits are hydraulically connected.
Known solutions of hydraulic power brake systems with central control have an energy supply, a (safe) central electronic control and a drive, which is responsible for the brake-pressure build-up. Some configurations of such brake systems have a hydraulic energy accumulator (high-pressure accumulator). However, such types of construction have proven to be inefficient in terms of energy, so that so-called power on demand functionalities are preferably used. In such a case, the dependence of the corresponding components on the electrical energy supply or the electrical system of the vehicle has to be accepted.
In order to meet the requirements of ECE 13, such brake systems must have a hydraulic-mechanical fallback level, where the driver can brake the vehicle with a deceleration of at least 0.3 g by physical strength in the event of failure of the primary functions of the brake system. However, a disadvantage of this is that in particular cases there is only limited effectiveness of this fallback level for the vehicle user. When the fallback level comes into effect, the driver often has the perception that there is a total failure of the brakes, since the feel of the pedal and the exertion of force necessary for braking as well as the maximum deceleration differ significantly from those in normal operation of the brake system. There is therefore usually a very great difference between the full function and the fallback level of the brake system in respect of brake-pedal force/travel and deceleration. In addition to this there is a relatively great probability of activation of the fallback level during normal vehicle operation being caused by the required electrical/electronic components, in particular in comparison with systems with normal braking function that are based on purely hydraulic/mechanical components, such as for example systems with a vacuum booster. For the driver who has become accustomed to the normal function of the brake system and has developed corresponding automatic responses, there is the risk that, in a situation where there is a failure of the normal function of the brake system, he cannot adapt himself quickly enough, so that an extremely hazardous situation may arise for the driver and his vicinity.